1. Field of the Invention
The present invention is concerned with novel pyridine derivatives, liquid cyrstalline mixtures which contain these pyridine derivatives as well as their use for electro-optical purposes.
2. Description
Liquid cyrstals are used primarily as dielectrics in indicating devices, as the optical properties of such substances can be influenced by an applied voltage. Electro-optical devices based on liquid crystals are well-known to the person skilled in the art and can be based on various effects such as, for example, dynamic scattering, the deformation of aligned phases (DAP cells), the Schadt-Helfrich effect (twisted-nematic cells), the guest/host effect (guest/host cells). Most of the commercial indicating devices are based on the Schadt-Helfrich effect with a twisted nematic structure.
The liquid crystals which are used must have a good chemical and thermal stability and a good stability towards electrical fields and electromagnetic radiation. Further, the liquid crystals should be colourless, should have short response times and low viscosities, should give a high contrast and should possess a suitable mesophase, for example a nematic, cholesteric or chiral smectic, preferably a smectic C, F or I, phase in the usual operating temperatures. As liquid crystals are usually used as mixtures, it is, moreover, important that the components have a good miscibility with one another. Further properties, such as, for example, the electrical conductivity, the threshold potential, the dielectric anisotropy and the optical anisotropy must fulfill different requirements depending on the type of cell which is used.
Indicating devices having a high density of information (e.g. in computer terminals) have recently been used to an increasing extent. In order to keep the required connections small, the multiplex control has been further developed. Liquid crystals having improved multiplexibility and at the same time steep transmission curves are, however, required for this purpose.
The indicating devices mentioned earlier generally have response times in the order of several milliseconds or more.
In order to improve the response times there have also recently been used liquid crystals having ferroelectric properties. Chiral smectic phases, preferably smectic C phases, are used in this application. Hitherto, however, only a few of such liquid crystals have been known and the mesophase ranges are mostly narrow or lie at a relatively high temperature. Further, the stability of these liquid crystals is often unsatisfactory.